Numerous capacitance-type measuring devices for making linear and angular measurements have been developed wherein two support members or scales, on which are respectively mounted arrays of discrete, capacitively coupled electrodes, are displaced relative to each other, and the relative positions of the two scales are determined by sensing the resulting change in the capacitance pattern created by the arrays of electrodes. Typically, the capacitance pattern is sensed by applying a plurality of periodic signals to one of the electrode arrays and measuring the shift in the signals resulting from the transfer to the other array of electrodes. Such measuring devices have a broad range of applications, from large-scale measuring devices such as three-dimensional coordinate measuring systems and numerically controlled finishing machines, to small-scale devices such as portable calipers, micrometers and the like.
Although capacitance-type measuring devices have become increasingly popular, they have heretofore been subject to a number of disadvantages which have limited their wider application. A major source of disadvantages has been the fact that conventional capacitive-type measuring devices have typically only been capable of making relative, and not absolute, measurements. That is, measurements are typically made by sensing the relative change in position of the scales with respect to a reference position, which requires a continuous sensing of the change in the capacitance pattern created by the electrode arrays so that repetitions of the pattern can be counted. Further, relative measurements require that a new reference or zero position be established before every measurement, which makes such devices relatively inconvenient to use.
In addition, the rate at which the scales of relative measurement devices can be displaced with respect to each other is limited by the speed of the signal processing which can be accomplished. On the one hand, if the scales are displaced too quickly, miscounting can occur. On the other hand, increasing the allowable scale displacement speed entails the use of high frequency signals and sophisticated signal processing circuitry, which substantially increases the cost of the measurement devices.
The ability to make absolute measurements of scale position, i.e., measurements based solely on the respective final measurement positions of the scales, obviates the various problems discussed above. A zero or reference setting for the scales can be established during assembly of the measuring device, and there is no need to adjust the setting during subsequent measurements. Nor are there any limitations on the displacement speed of the scales, since the capacitance pattern between the scale electrodes needs to be sensed only at the final measurement position of the scales. Further, the power source needs to be connected only when the final measurement position is to be measured, which greatly reduces the power consumption and allows even small-capacity power sources, such as solar batteries, to be employed.
The present inventor has previously developed a capacitive-type measuring device capable of making absolute measurements, which is shown in FIGS. 10 and 11 of U.S. Pat. No. 4,420,754 ("the '754 Patent"). This device utilizes separate first and second pairs of transmitter/receiver electrode arrays arranged in side-by-side relationship. The relationship of the pitch of the transmitter electrodes to the pitch of the receiver electrodes is the same in each array pair, but the respective transmitter/receiver electrode pitches in the two array pairs differ slightly. Two separate n-phase signals are applied to the respective transmitter electrodes of the two array pairs and two independent signals V.sub.1 and V.sub.2 are obtained (via associated transfer and detector electrodes) from the respective receiver electrodes in each array pair. An absolute measurement value is derived by measuring the phase difference between the two signals V.sub.1 and V.sub.2.
However, the utility of the '754 Patent measuring device is limited. For example, because the absolute measurement value calculation is based on two independent measurements, slight errors in either measurement accumulate and can result in large position measurement errors. Consequently, the respective signal processing circuits must have precisely matched performance characteristics in order for accurate absolute position measurements to be obtained. Further, if the two measurements are not made at precisely the same time, even very slight displacement of one support member relative to the other between the times the two measurements are taken can cause very large errors in the position measurement.
In addition, the physical requirement of two separate pairs of electrode arrays in the measuring device of the '754 Patent limits its application in hand measuring tools, which must have a compact size. Further limiting its application in portable measuring devices is the increased power consumption caused by the requirement for duplicate signal processing circuitry.